1. Field of the Invention
The present invention relates to an image forming apparatus of an electrophotographic method that forms an electrostatic latent image by scanning a photosensitive drum with a laser beam and that causes a toner to adhere thereto to record an image on a recording medium.
2. Description of the Related Art
Conventionally, an image forming apparatus operating by an electrophotographic method has been widely used, by which an electrostatic latent image (electrostatic image) is formed on a photosensitive member by exposing to light an electrophotographic photosensitive member that is a charged image bearing member, according to image information, and then an image is formed by developing the electrostatic latent image with a developer.
In addition, as discussed in Japanese Patent Application Laid-Open No. 2002-23435, there is a back area exposure method, which exposes to light a non-image area (bright image area) where a developer is not applied, in an image forming apparatus of an electrophotographic method. The back area exposure method is a method in which little unevenness appears in an image area (dark image area) where the developer is applied, and fogging (phenomenon in which the developer is deposited on the non-image area) and change in density occur infrequently.
As to a toner used as a developer in the electrophotographic method, a negative toner, in which the normal charging polarity is negative, is currently mainstream from viewpoint of the stability of material. When using the negative toner, the back area exposure method is also effective against defocusing of image that occurs due to a dull electrostatic latent image. The defocusing of image due to the dull latent image occurs when NOx is deposited on the surface of the photosensitive member, and a surface electrical resistance of the photosensitive member decreases. NOx is formed when ozone produced in charging a photosensitive member combines with nitrogen in air.
When the back area exposure is performed using the negative toner, the charging polarity of the photosensitive member is a positive polarity. The amount of ozone produced when the photosensitive member is charged with a positive polarity is approximately one-fifth of that produced when the photosensitive member is charged with a negative polarity. Therefore, in the back area exposure method, the surface resistance of the photosensitive member decreases much less than in an image area exposure method which exposes the image area where the developer is applied.
Additionally, as discussed in Japanese Patent Application Laid-Open No. 2002-258587, the back area exposure method is also effective against streaks when an amorphous silicon photosensitive member having a long lifetime is used. Such streaks may appear if the charging polarity of the photosensitive member and the charging polarity of the toner to be used are the same. Accordingly, in the back area exposure method, such streaks typically do not appear.
In recent years, image forming apparatuses for printers, copying machines, facsimiles, etc. have made remarkable progress in high resolution. Here, as an example, a case is considered where a resolution is increased from 600 dots per inch (dpi) to 1200 dpi.
In this case, a size of one pixel is halved from 42 μm to 21 μm. However, the spot diameter of an exposure device for forming a latent image of an image pattern on a charged photosensitive member is currently approximately 60 μm. Therefore, even if an image signal is varied for every 21 μm, the spot diameter remains at 60 μm. Accordingly, it is hard to reproduce an image pattern of, for example, a thin-line such as the one consisting of 2 pixels or isolated dots (hereinafter referred to as “thin-line portion”).
As illustrated in FIG. 7, in a thin-line portion, a formed latent image is shallower in depth compared to an image pattern showing, for example, a plurality of consecutive pixels (hereinafter referred as “solid portion”). Accordingly, electric field strength in a development unit becomes different between the thin-line portion and the solid portion, and the amount of applied toner becomes smaller in the thin-line portion. Therefore, reproducibility in the thin-line portion may be inferior to that in the solid portion. Such a phenomenon that a latent image becomes shallow is more remarkable in a back area exposure method than in an image area exposure method. In the back area exposure method, “a latent image becomes shallow” means that the electric potential (absolute value) of an image area becomes lower.
In order to address this problem, narrowing the spot diameter of an exposure device is considered. However, when the spot is optically narrowed, a ratio of change of a spot diameter increases against change of focal distance. As a consequence, an adjustment range of focal point becomes limited which makes the adjustment difficult. In addition, there arises a problem that the size of dots to be formed changes only by a slight vibration. While there are a mechanism to automatically correct focal points, and a method for shortening exposure wavelength, they cause the apparatus cost to increase.
Furthermore, there is a method for setting the charging potential of the photosensitive member at a high level, which emphasizes the reproducibility of the thin-line portion. However, in this case, the amount of applied toner in the solid portion increases more than necessary, and problems such as increase in consumption of the toner and flying of the toner are more likely to arise.